A variety of bonders are depicted in "Semiconductors handbook (Revised Edition)", Ohm Publishing, for electrically connecting an electrode of an IC chip and a conductor of its package by the wire bonding technology. The inventors of the present application have discovered that such a conventional bump bonder for wire bonding can be utilized for forming a bump on an electrode of an IC chip for electric connection, and have devised such a bump bonder as the one proposed in Japanese Patent Application No.8-249724. This bump bonder includes at its primary section or bonding station a bonding stage equipped with a heater for ultrasonic thermal compression bonding and a bonding head therebehind supported for movement in both X and Y directions.
As shown in FIG. 4, the bonding head 1 comprises a wire reel 3 mounted to a wire reel mount 2 and a bonding mechanism 10 for carrying out bonding operation with a wire 4, which is fed from the wire reel 3 located above, to a bonding object such as an IC chip or a wafer placed on the bonding stage (not shown). Also provided between the wire reel 3 and the bonding mechanism 10 are a first wire tensioner 7 for applying an upward blow of air 6 to a midway portion of the wire 4 to cause the wire 4 to be curved upwardly and a second wire tensioner 9 for exposing the wire 4 to an upward blow of air 8 just above a wire guide 11a of a damper 11 in the bonding mechanism 10 to tension the wire 4 upwardly. Accordingly, the wire 4 drawn from the wire reel 3 can constantly be fed into the bonding mechanism 10 while being floated under proper tension through a prescribed feeding path by the blows of air 6 and 8.
Referring to FIGS. 4 and 5, the bonding mechanism 10 comprises the damper 11 for securely holding the wire 4, a horn 13 provided with a capillary 12, through a distal end of which the wire 4 is passed, for applying ultrasonic vibration via the capillary 12 to a ball 5 (FIG. 6) developed from a leading end of the wire 4, and a torch 14 for forming the ball 5 with electric discharge. A monitor camera 15 is provided above for visually monitoring the bonding operation, which displays a monitoring image on a monitor display (not shown), and delivers a monitor signal to a data processor (not shown) where it is processed.
The damper 11 and the horn 13 are mounted to a swing member 16 for upward and downward swing movements about an unshown pivot. The swing member 16 is driven to swing by a vertical motion electromagnetic driver 17 for causing integral vertical movements of the damper 11 and the horn 13. Also, an open/close electromagnetic driver 18 is provided for opening and closing the damper 11.
The bonding operation of the above arrangement will now be explained referring to FIG. 6. While the damper 11 and the capillary 12 are located at their initial levels with the leading end of the wire 4 extending a given length from the lower end of the capillary 12, the torch 14 is activated to produce a spark current in synchronism with the movement of the bonding head 1 to a location where it comes opposite to a target electrode 19 on an IC chip 20 so that the leading end of the wire 4 is melted to form a ball 5 such as shown in FIG. 6(a). The opposite relationship between the wire 4 and the electrode 19 is monitored and controlled precisely with visual recognition by the monitor camera 15.
The swing member 16 is then driven by the vertical motion electromagnetic driver 17 for causing the damper 11 and the capillary 12 to descend towards the IC chip 20 placed on the bonding stage (not shown). When the capillary 12 presses down the ball 5 of the wire 4 against the electrode 19 on the IC chip 20, as shown in FIG. 6(b), the horn 13 applies ultrasonic vibration via the capillary 12 to the ball 5 which is thus bonded to the electrode 19 by thermal compression and ultrasonic vibration. It is preferable to set the compression force within the range of 30 to 50 g and the ultrasonic vibration, which is applied in a horizontal direction, to be 0.5 micrometer in amplitude and 60 to 70 kHz (specifically 63.5 kHz) in frequency.
During the bonding operation, the clamper 11 is opened by the open/close electromagnetic driver 18 and lifted up together with the capillary 12 to a prescribed level by the vertical motion electromagnetic driver 17. Accordingly, the wire 4 is extended from the lower end of the capillary 12 by a certain length as shown in FIG. 6(c).
The clamper 11 is then closed by the open/close electromagnetic driver 18 to hold the wire 4 securely, after which the swing member 16 is driven by the vertical motion electromagnetic driver 17 to cause the clamper 11 and the capillary 12 to move upwards as shown in FIG. 6(d) to their initial level shown in FIG. 6(a). In an early stage of this step, the wire 4 is separated at a boundary of heat effect hence leaving on the electrode 19 a bump 21 of about 60 micrometers high which consists of a ball portion 21a and a wire portion 21b of 30 to 40 micrometers high.
Since the above-described conventional bump forming method includes the step of drawing out the wire 4 shown in FIG. 6(c), which requires substantially 0.02 second, the cycle of bump forming operation from the start of action until it is returned to the initial state takes a total of about 0.1 second, because of which productivity cannot be further improved.
It is thus an object of the present invention to provide a bump forming method and a bump bonder with which the time required for the cycle of bump forming operation can be reduced to thereby improve the productivity.